If you're talking about the Preview Pane or Reading Pane, you can turn it
off via the View menu.  Your messages aren't really being "opened" in this
pane -- just previewed.  However, depending on your Outlook version and how
well (or not well) your copies of Windows/Internet Explorer are patched with
the latest updates, certain types of viruses can be triggered just by
previewing a message.  (Of course, if you don't have the preview pane on and
you open the message, you're no safer than if you had previewed it first --
so preview can actually be safer.)